The present invention relates to metal alloys, and, more particularly, to an ultrahigh carbon steel containing aluminum.
The simultaneous achievement of high strength, good ductility, microstructural stability and excellent workability are continuing objectives in the search for improved steels. While the first three properties are sometimes obtained, the compositions and microstructures needed to obtain these properties often involve particles or other microstructural features which preclude excellent workability. Steels are usually cast as thick sections and reduced by rolling or forging steps. If the steel is insufficiently workable, it may develop cracks during reduction which render the final product unacceptable. It is also essential for a commercial steel that the desired properties be obtained with relatively inexpensive alloying additions and through processing steps which are straightforward and compatible with existing steel mill processing techniques.
The selection and processing of steels also requires due consideration of the end use of the steel. In many applications a uniform, fine-scale microstructure is known to be a necessity. In particular, the manufacturing technique of superplastic forming has received widespread attention, because in many cases parts may be formed to essentially their final shape in a single step. Material costs and costs of secondary processing such as machining may therefore be significantly reduced. Superplastic behavior is usually found in metals having very fine grain sizes at elevated temperatures, and is marked by a high sensitivity of the stress to strain rate during deformation.
The selection of alloying additions and processing procedures therefore requires consideration of the fabrication technique, as well as the ultimate properties needed in the finished end product. Conventionally processed materials require acceptable workability during fabrication. The requirements in specialized processing operations such as superplastic forming are even more stringent.
To prepare an alloy for a superplastic forming operation, the alloy must first be reduced in section and processed to a fine grain structure. Although in some cases superplasticity is not related to grain size, in most instances a finer grain size results in increased superplastic strain rate for any selected stress level. Most alloys must therefore first be processed to a fine grain size which is stable when the alloy is heated for superplastic forming. If the fine grain size is not sufficiently stabilized, the grains coarsen so much during the superplastic forming operation that the superplastic characteristic is lost before forming is completed, and the forming operation fails. Thus, stabilization of fine grain structures and increased superplastic forming rate are keys to improving superplastic fabrication operations.
Most of the commercial-scale applications of superplastic forming have utilized titanium, nickel, and aluminum alloys of interest in the aerospace industry. Iron-based superplastic alloys have also been developed, including, for example, the ultrahigh carbon steel disclosed in U.S. Pat. No. 3,951,697. This patent relates to a process for preparing a hypereutectoid steel having a fine grain size and an array of fine iron carbides to stabilize the fine grain size during subsequent superplastic processing. The superplastic forming is then accomplished just below the eutectoid (or A.sub.1) temperature of about 725.degree. C., since the steel does not exhibit the desirable superplastic property below about 600.degree. C. or above about 750.degree. C.
While the ultrahigh carbon steel represents a significant advance in the art, problems remain in its economic application on an industrial scale. When the steel is heated to the warm and hot working range, the fine iron carbides tend to coarsen, with the result that the fine grains also grow to larger sizes. Since a fine grain size is required for superplasticity, the growth of the grains may result in the loss of the superplastic property, even though the steel is heated to the appropriate temperature range. The superplastic forming operation must be completed before the grains grow too large. In some cases, the processing cannot be completed because the grains coarsen to a size such that superplasticity is lost, thereby making the superplastic forming operation commercially impractical.
An important consequence of the increase in grain size during heating in superplastic processing is a reduction in the allowable superplastic forming strain rate. Studies and calculations have shown that an increase in grain size from about 1 micrometer to about 5 micrometers can be expected to reduce the superplastic strain rate at constant stress by about a factor of 100. Since a high strain rate results in a short forming time, grain size coarsening is expected to increase drastically the time required to form a part.
One approach to an improved ultrahigh carbon steel, wherein additions of silicon and a carbide stabilizing element are made, is described in U.S. Pat. No. 4,533,390. The ultrahigh carbon steel containing silicon and a carbide stabilizer may be processed to include a stable array of iron carbide particles which act to retain the fine grain size during subsequent processing, and to increase the eutectoid temperature. The result is that superplastic processing of this material may proceed at higher strain rates and lower stress levels than used for plain carbon ultrahigh carbon steels. This steel provides an important advance, but has limitations in practical application. For higher contents of silicon, hot and warm working of such steels becomes difficult due to edge and surface cracks which occur during processing. The ductility of such steels is also limited at ambient temperatures, with cracks appearing after about 20 percent reduction in rolling.
Consequently, there has been a need for an improved iron-based alloy having enhanced ductility during hot, warm and cold working, as well as a stable, fine grain size at elevated temperatures for superplastic formability. Such improved ductility is important both in the end use and also in the processing operations required to reduce the thickness and produce the fine, stable grain size. Desirably, such an alloy would also have increased superplastic forming strain rates to enhance the economics of commercial superplastic forming operations. The present invention fulfills this need, and further provides related advantages.